The lowest power test bed of the Center for Compact and Efficient Fluid Power (CCEFP) is directed towards developing technologies for human assist devices. The platform for this test bed has been developing portable fluid powered orthoses. There is a need in physical medicine and rehabilitation to create orthotic devices that better meet the needs of persons with limb weakness or paralysis. These needs include the ability to create torque about the ankle to support the foot bidirectionally while walking.
The portable powered ankle-foot orthosis (PPAFO) is a pneumatic device that has been created to meet the need for bi-directional torque assistance. One of the novel features of the PPAFO is its portable nature, being powered by a pneumatic power source, commonly a compressed CO2 bottle. The portable nature of the power supply allows for the user to wear the PPAFO in any setting, away from laboratory tethers and testing equipment.
One of the limitations of the portable aspect is the finite amount of fuel that can be stored in the portable compressed CO2 bottle. The finite fuel results in a limited run time for the PPAFO system. Consistent with one of the main goals of the CCEFP, which is to improve the efficiency of fluid power, an exhaust gas recycling scheme for the PPAFO was developed.
This pneumatic recycling scheme was developed to capture the exhaust from one actuation direction and re-route the captured exhaust to power the reverse actuation direction. In the application of the PPAFO, this scheme works incredibly well based on the specific power needs for gait assistance.
During walking, the ankle generates more torque in the plantarflexion (toes down) direction, than in the dorsiflexion (toes up) direction. The pneumatic system was designed to regulate the amount of torque applied based on the direction of assistance. In the original pneumatic system design, the plantarflexion torque was regulated by the CO2 bottle pressure regulator, and for the dorsiflexion torque, a secondary in-line pressure regulator was used to further down-regulate the pressure since less torque is necessary to hold up the foot and toes during swing. Both directions of actuation were powered from the portable finite CO2 bottle. In the recycling scheme, the plantarflexion actuation is powered from the CO2 bottle and regulated by the bottle regulator which is the same as the original pneumatic design.
The recycling occurs when the exhaust from the plantarflexion actuation is captured into an accumulator. Then instead of powering the dorsiflexion actuation from the CO2 bottle and down-regulating the pressure to reduce the torque, the dorsiflexion actuation is powered from the accumulated exhaust—which is already at a lower appropriate pressure for dorsiflexion actuation. Implementing the pneumatic circuit with recycling to power the PPAFO improved the overall system efficiency by 75% from 27 to 47 minutes, allowing for longer use of the PPAFO without the need to refill or replace the fuel supply.
Contributed by Ms. Morgan K. Boes, MD/PhD candidate at the University of Illinois, Urbana-Champaign.
Filed Under: Hose • wraps + sleeves, Pneumatic equipment + components